Internal combustion engines, such as gasoline engines, commonly employ a four-stroke working cycle. The four strokes may be referred to as the intake, compression, combustion (power), and exhaust strokes, which occur during two crankshaft rotations per working cycle of the engine. The working cycle may be understood to begin with the intake stroke with a piston at Top Dead Center (TDC) position, when the piston is closest to the cylinder head and farthest away from the axis of the crankshaft. A stroke may be understood to refer to a full travel of the piston from Top Dead Center (TDC) position to Bottom Dead Center (BDC) position, when the piston is furthest from the cylinder head and closest to the axis of the crankshaft.
During the intake stroke, the piston may be understood to descend from the top of a cylinder (e.g., TDC) to the bottom of the cylinder (e.g., BDC), reducing the pressure inside the cylinder. During the travel of the piston, an intake valve of the cylinder may open and a mixture of air and fuel may be introduced into the combustion chamber of the cylinder, under atmospheric or greater pressure, through an intake port. The intake valve may then close.
Thereafter, during the compression stroke, the piston may then return to the top of the cylinder, compressing the air-fuel mixture in the combustion chamber. Once the piston returns to TDC, the crankshaft will have undergone the first rotation of the working cycle. Next, the power stroke may be understood to begin when the piston is at TDC. After igniting the compressed air-fuel mixture with an igniter, such as a spark plug, the resulting pressure from the combustion of the compressed air-fuel mixture may then force the piston back down towards BDC. This stroke is the main source of the engine's torque and power.
The compressed air-fuel mixture within the combustion chamber may be ignited by the igniter near the end of the compression stroke. Igniting the air-fuel mixture before the piston reaches TDC may allow the resulting flame to better propagate and the mixture to more fully burn soon after the piston reaches TDC. However, if the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture may push against the piston as it is moving up during the compression stroke, causing possible engine damage. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure may occur during the combustion stroke after the piston has traveled too far down the cylinder. This often results in lost power, high emissions, and unburned fuel.
After the combustion stroke, and during the exhaust stroke, the piston once again returns to TDC while an exhaust valve of the cylinder may be opened. This action may evacuate the products of combustion from the combustion chamber of the cylinder by pushing combustion products through an exhaust port. The exhaust valve may then close. Once the piston returns to TDC, the crank shaft will have undergone the second rotation of the working cycle and the engine will thereafter repeat the cycle.
In certain situations, the internal combustion engine may exhibit abnormal combustion. Abnormal combustion in a spark-initiated internal combustion engine may be understood as an uncontrolled deflagration occurring in the combustion chamber as a result of ignition of combustible elements therein by a source other than the igniter. One particular example of abnormal combustion may include pre-ignition. Pre-ignition may be understood as an abnormal form of combustion resulting from ignition of the air-fuel mixture prior to ignition by the igniter. Anytime the air-fuel mixture in the combustion chamber is ignited prior to ignition by the igniter, such may be understood as pre-ignition.
In some instances, pre-ignition occurs during high speed operation of an engine when a particular point within the combustion chamber of a cylinder may become hot enough to effectively function as a glow plug (e.g. overheated spark plug tip, overheated burr of metal) which can provide a source of ignition thus potentially causing the air-fuel mixture to ignite before ignition by the igniter. Such pre-ignition may be more commonly referred to as hot-spot pre-ignition, and may be inhibited by simply locating the hot spot and eliminating it.
More recently, vehicle manufacturers appear to have observed intermittent abnormal combustion in their production of turbocharged gasoline engines, particularly at low speeds and medium-to-high loads. More particularly, when operating the engine at speeds less than or equal to 2,000 rotations per minute (RPM) and under a load with a break mean effective pressure (BMEP) of greater than or equal to 10 bars, a condition which may be referred to as low-speed pre-ignition (LSPI) may occur in a very random or otherwise stochastic fashion. Unlike hot-spots that cause pre-ignition, LSPI is not necessarily tied to a particular location within an engine, and thus has raised numerous non-trivial challenges in the context of turbocharged gasoline engines.